Nanoemulsions have many physical properties that distinguish them from other emulsions. Due to their small mean droplet size, which is often smaller than optical wavelengths of the visible spectrum (thus less than about 400 nm), nanoemulsions usually appear transparent or translucent to the naked eye, even at high droplet volume fractions. The terms sub-micron emulsion (SME) and miniemulsion are sometimes used as synonyms for the term nanoemulsion. Nanoemulsions have great potential for use in many industries and applications.
A nanoemulsion may be defined as a type of emulsion wherein the dispersed/discontinuous phase has a mean droplet size of less than 1000 nm; the components of the continuous and dispersed/discontinuous phases must be immiscible enough to allow for the respective phase formation. Some nanoemulsions may have a smaller range for mean droplet size specified, and it is possible to have more than one dispersed/discontinuous phase. These emulsions are typically composed of a nonpolar phase (usually denoted as the oil phase), a polar phase (typically aqueous and denoted as the aqueous or water phase), a surfactant and optionally one or more additional co-surfactant(s). There may be a narrow droplet size distribution depending on the preparation process.
Nanoemulsions are usually stable against sedimentation or creaming, with high kinetic stability, probably because Brownian motion and diffusion rates are higher than the sedimentation or creaming rates induced by gravity. However, they are usually non-equilibrium systems (typically requiring energy input for formation), and thus thermodynamically unstable, and therefore have a tendency to separate into the constituent phases.
In general, there are two primary methods to prepare a nanoemulsion: (1) by “persuasion” and (2) by “brute force”, which are described in the chapter, “Nanoemulsions”, by Salager, Forgiarini and Marquez, in Pharmaceutical Emulsions and Suspensions, (2nd edition), Nielloud and Marti-Mestres editors, Taylor and Francis (London). Preparation by persuasion involves taking advantage of certain phase transitions, while preparation through brute force involves imparting sufficient shear to reduce the droplet size of the immiscible internal phase below 1000 nm. These methods may be further described as follows.